This invention relates to a bias voltage supply for controlling an adjustable focus x-ray tube. The new bias supply is especially useful for controlling x-ray tubes in which the focal spot is reduced below the size normally available in conventional x-ray tubes to permit diagnostic procedures requiring high resolution radiography.
Conventional x-ray tubes used for medical diagnostic purposes are customarily provided with means for selecting the operating characteristics such as the anode to cathode voltage, the electron beam current, the focal spot size and the conduction or exposure time interval. The x-ray tube usually has an electrode, called a grid or a cup, to shape the electric field around the electron emitting filament for controlling the beam cross section so that a suitable focal spot is formed on the anode target. The control electrode is operated at cathode potential for full electron beam current and is biased negatively when electron beam cut-off is desired. The operating parameters of the tube, that is the maximum permissible combination of anode voltage, beam current and exposure time interval are normally coordinated in a manner that prohibits loading the target to the extent that melting might occur in the focal spot region of the target. Typically, the minimum focal spot size is limited to about 1.2 mm for conventional radiography.
For some diagnostic procedures such as brain or neurological examinations, the radiologist must be able to distinguish very fine details in the x-ray image so a high resolution radiography procedure must be adopted which means that a finer focal spot than is obtainable in a conventional general purpose x-ray tube must be used. High resolution or microfocus x-ray tubes have been developed for this purpose. These tubes have a second electrode intervening between the first electrode and the anode target for augmenting focusing. In some prior art systems, the second control electrode is connected to a variable bias voltage supply which permits making the second control electrode less negative, but not positive, with respect to the cathode for increased focusing. Unfortunately, as focus is adjusted in such tubes, electron beam current also changes undesirably and unpredictably. Another disadvantage of prior art systems results from the focusing electrode bias voltage being derived from a source that is independent of the cathode voltage of the tube so bias and, hence, the amount of focusing may vary independently with power line fluctuations and other transients during x-ray exposures which may endure for 1 millisecond to 6 seconds, by way of example.
U.S. Pat. No. 3,916,202, owned by the assignee of this application, discloses a variable microfocus x-ray tube wherein the second control or focusing electrode may be biased positive with respect to the cathode to obtain maximum focusing without having current flow to the control electrode as would be expected if positive bias voltage is used. This patent discloses use of two independent bias supplies which are connected through a two-pole switch to the second and first control electrodes, respectively such that when the tube is being used for making an x-ray exposure, a positive bias voltage is applied to the second focusing electrode and the focusing cup is connected to the filament and is at the same potential. To terminate an exposure, the positive supply is disconnected by the switch and the negative bias voltage supply is connected to both the first and second control grids to make the tube nonconductive. However, during an exposure interval, with the bias voltages derived from a source independent of the voltage applied to the x-ray tube, the tube is still vulnerable to focal spot size variations because of the second control grid voltage failing to track variations in the voltage applied between the anode and cathode of the tube which might result from power line voltage fluctuations, transients, impedance changes with load, and waveform variations.